The present invention relates to a friction clutch, especially for a motor vehicle, and more particularly relates to a clutch having a wear compensating device, for taking up wear in, in particular, the friction liners. Such a device can be called a wear take-up or wear compensating device.
A conventional friction clutch generally includes a reaction plate, which may be in two parts to define a damped flywheel or a flexible flywheel, which is mounted in rotation on a first shaft, typically a driving shaft such as the crankshaft of the internal combustion engine of the vehicle, and supporting by its outer periphery a cover plate to which at least one pressure plate is attached.
The pressure plate is fixed with respect to the cover plate and reaction plate for rotation with them, while being displaceable axially under the biasing action of resilient clutch engaging means acting axially and controlled by clutch disengaging means. The clutch engaging means may consist of helical springs or one or a pair of Belleville rings mounted in series or in parallel and subjected to the action of declutching levers constituting the declutching means. Generally, the means for engaging and disengaging a clutch consist of the same component, for example a metallic diaphragm bearing on the cover plate. The diaphragm may be mounted in series or in parallel with a Belleville ring in order to provide assistance to the declutching force, in the manner described for example in French patent specification FR 2 753 758A.
A friction disc, carrying friction liners at its outer periphery and fixed to a shaft for rotation with the latter, this shaft being typically a driven shaft such as the input shaft of the gearbox of the vehicle, is interposed between the pressure plate and the reaction plate in such a way as to be gripped between them when the clutch is in its engaged state. The clutch engaging means control the axial displacement of the pressure plate when they are actuated by a clutch release bearing through the declutching means.
During the working life of such a clutch, the friction liners, as well as the backing or counter elements consisting of the pressure plate and reaction plate, become worn. This causes variation in the position of the pressure plate and the positions of the axially acting clutch engaging means and clutch release bearing. One consequence of this is that the force with which the friction disc is gripped between the pressure and reaction plates is subject to variation because the working conditions of the clutch disengaging means are modified. Another consequence is that the force necessary to disengage the clutch is affected. By giving such a clutch a wear compensating device, these drawbacks are avoided, with the clutch engaging means and the clutch release bearing, which is typically in constant engagement on the declutching means, occupying the same position when the clutch is in its engaged condition.
A friction clutch equipped with a wear compensating device is so arranged that one of the abutments of the clutch engaging means, whether this is the abutment which is connected directly or indirectly to the cover plate or whether it is the one connected directly or indirectly to the pressure plate, is displaceable axially. A so-called compensating means is provided for carrying out this displacement, in association with a so-called actuating means which actuates the compensating means, and a displacement means which itself controls the actuating means by authorising (or otherwise) the operation of the actuating means. A wear compensating device is of course controlled by a so-called detecting means which senses wear in the liners, directly or indirectly. In other words, it senses, for example, the position of at least one of the components of the clutch that is influenced by such wear, with the detecting means actuating the compensating means or the actuating means or the displacement means.
Various types of wear compensating device are known. For example French patent specification No. FR 2 753 503A describes a clutch in which the wear compensating device comprises ramp means disposed circumferentially and constituting the compensating means, being located axially between the displaceable abutment and the pressure plate and adapted to be driven in rotation by means of a set of teeth which they carry at their periphery and with which there cooperates a tangentially disposed worm. Means for driving the worm in rotation are provided, and are activated by wear in the friction liners, the worm and worm driving means being carried by a support which is fixed to the cover plate, and the elastic, axially acting clutch engaging means consisting of a diaphragm. The worm driving means are controlled by the diaphragm. The ramp means consist of an annular ring which defines the displaceable abutment of the diaphragm and inclined ramps which are spaced apart circumferentially, while the pressure plate has pads or ramps which are arranged to cooperate with the ramps of the ramp means itself. The worm is mounted for sliding movement along its axis and is subjected to the action of an elastic means or so-called take-up means, constituting the actuating means; and, when it is displaced along its axis it drives the ramp means in circumferential rotation.
In the same clutch, the worm driving means consist of a ratchet wheel fixed to the worm and associated with a pawl which cooperates with the ratchet wheel. In this case, the detecting means is in the form of a tongue actuated by the diaphragm and adapted to cooperate with the ratchet wheel, its course of travel being increased with increasing wear, until, when the clutch is engaged, it is able to push a tooth of the ratchet wheel and cause the worm to turn, so compressing the elastic take-up means. The displacement means is represented by a diminution of the load on the area of engagement during the declutching operation, in which the elastic take-up means, when its load is sufficiently high to overcome friction forces, enables the inclined ramps to be displaced and therefore the pressure plate abutment to be displaced axially.
Thus, the detecting means, which in this case is the tongue controlled by the diaphragm, acts via the ratchet wheel on the actuating means which in this case is the elastic take-up means, thus producing an actuating force. The actual displacement takes place during a subsequent declutching operation.
In United Kingdom patent specification GB 2 294 983A, the clutch engaging means consist of a diaphragm and the displaceable abutment is again the pressure plate abutment. The detecting means consists of a pin which extends frictionally through the pressure plate and is in engagement on the reaction plate. The compensating means again consists of ramps. The actuating means is a spring which is in permanent engagement between the ramps and counter-ramps in the direction of the increase of axial thickness. Here again, the displacement is obtained during the declutching operation by a reduction in the load on the area of contact, while the pressure plate is displaced as a function of wear with respect to the pin, which carries an abutment in the form of a lever cooperating with the ramp means. The detecting means therefore acts on the compensating means.
In United Kingdom patent specification GB 2 294 301A, the clutch engaging means again consist of a diaphragm, the compensating means are ramps, and the actuating means is again a spring acting permanently between the ramps and (in this case) the cover plate. In this case the displaceable abutment is the primary abutment carried by the cover plate, the clutch being of the push to release type and the displacement being obtained by withdrawal or retraction of the secondary abutment which is in facing relationship with the elastically mounted primary abutment on the pressure plate side, the maximum declutching force increasing with wear. This constitutes the detection means which, by retracting, also constitutes the displacement means. The detecting means can therefore be said to act on the displacement means.
A clearance appears between the primary and secondary abutments during the retraction of the secondary abutment: this clearance is taken up by a movement of the primary abutment, effected by the actuating means.
In U.S. Pat. No. 5,564,541, in the version shown in FIG. 10 of that document, the detecting means is a pin which is arranged with a clearance between the cover plate and reaction plate, and which extends frictionally through the pressure plate, the force needed for displacing the pin with respect to the pressure plate being greater than the return force of the pressure plate, which in this case is due to the tangential tongues. The actuating means is a spring which acts permanently between the pressure plate and the ramps constituting the compensating means. It is the pressure plate abutment that is displaceable here, the clutch being of the pull to release type. In this case the course of travel varies in the declutching direction: the position of full disengagement of the clutch is fixed so that the displacement occurs at the end of the declutching operation, after wear has been detected and after the appearance of a clearance which is then taken up under the action of the actuating means.
In French patent specification FR 2 599 446A, in the version shown in FIG. 5 of that document, the elements are similar to those in the above mentioned United States patent, except that the compensating means is an axial one-way free wheel, and the actuating means is a spring which permanently biases the balls into engagement against their ramps.
In the version shown in FIG. 3 of U.S. Pat. No. 5,564,541, the detecting means is a pin which is arranged with a clearance between the cover plate and the pressure plate and extending frictionally through the cover plate. The force needed to displace the pin with respect to the cover plate is greater than the return force of the pressure plate, which in this case is due to the tangential tongues. The actuating means is a spring which acts between the cover plate and an annular rotary ring having ramps arranged to cooperate with counter ramps or backing ramps. It is accordingly the cover plate abutment that is displaceable in this case, the clutch being of the pull to release type. The displacement is produced at the end of the declutching operation, after wear has been detected and after the appearance of a clearance which is taken up by the actuating means. As in FIG. 10, the configuration on full declutching is fixed, the clutch engaging means including declutching levers associated with helical springs which bear on the cover plate.
As mentioned above, the wear take-up device ensures for the clutch engaging means (for example the diaphragm) the same engagement position regardless of the state of wear, namely the position which it occupied when the liners were new and the clutch was being fitted on the vehicle. It is of course important that this so-called nominal position corresponds to the desired nominal load exerted by the diaphragm on the pressure plate when the clutch is engaged. The position of the diaphragm, or in practice its inclination, is clearly fixed by the abutments, primary and secondary, on which it bears through its outer part, which is a Belleville ring. The primary abutment is carried by the cover plate and the position of the secondary abutment is a function of the positioning of the ramp means with respect to the counter ramp means. Positioning is ensured by the adjusting function. In a clutch of the kind described in French patent No. 2 753 503, it is ensured by the assembly consisting of the ratchet wheel, worm and control tongue adapted to cooperate with the control abutment.
This assembly is comprised in a cassette which has a specific support member for mounting the assembly therein.
For the nominal positioning of the diaphragm, the mechanism is associated with a notional friction disc, and a certain number of clutch engaging and disengaging operations are carried out, until the control tongue in engagement on the control abutment no longer jumps over a tooth of the ratchet wheel, the secondary abutment of the diaphragm being then correctly positioned.
As has been seen, the clutch mechanism is constructed by assembling together various components. These components are made, and are assembled together, with certain tolerances. This affects the precision of the relative positioning of the primary abutment and control abutment when these latter are formed on two different components as in the case in the above mentioned document, in which the primary abutment is carried by the cover plate and the control abutment by the support assembled to the cover plate.
In French patent application No. 98 16547 filed on Dec. 29, 1998, it was proposed to form the control abutment on the cover plate, like the primary abutment means.
It is thus easy to position the control abutment accurately with respect to the primary abutment means, especially when the primary abutment means are made in the form of circumferential press-formed elements formed in the base of the cover plate, the primary abutment means and the control abutment being formed simultaneously during fabrication of the cover plate.
The object of the present invention is to obtain in a simple way the accuracy required in the nominal position of the clutch engaging means, regardless of the type of primary abutment means or wear compensating device, and therefore regardless of the type of clutch mechanism involved.
According to the invention, a friction clutch, in particular for a motor vehicle, of the kind comprising a reaction plate adapted to be mounted on a driving shaft for rotation with the latter, a friction disc carrying at its outer periphery friction liners and adapted to be mounted on a driven shaft for rotation with the latter, a pressure plate, a cover plate fixed on the reaction plate, axially acting clutch engaging means which are controlled by declutching means, and which act between firstly the cover plate and secondly the pressure plate through primary abutment means and secondary abutment means respectively, the pressure plate being fixed to the cover plate for rotation with the cover plate while being displaceable axially with respect to the cover plate, and being subjected to the action of resilient return means biasing the pressure plate axially towards the cover plate, the said clutch further including a wear compensating or wear take-up device, with one of the abutments of the clutch engaging means being displaceable axially and the wear take-up device including a compensating means which is brought into operation by an actuating means adapted to effect the said displacement, the said actuating means being controlled by a displacement means in association with a detection means sensitive to wear in the liners, acting on at least one of the elements consisting of the actuating means, compensating means and displacement means, is characterised by the fact that adjustment means are provided for adjusting the nominal position of the clutch engaging means defined by the axial distance between the primary and secondary abutments, with the clutch in its engaged position.
Further features of the invention are defined in the dependent Claims of this Application.